1. Field of the Invention
The present invention relates to tool clamping/unclamping devices used for replacing a tool in a machining center or other various types of machine tools. More specifically, the invention relates to a tool clamping/unclamping device of the so-called nut runner system in which a tool holder is fixed at the main shaft of a machine tool with lock nuts.
2. Description of Related Art Statement
This type of tool clamping/unclamping device conventionally available is disclosed, for example, in Japanese Patent Publication No. 4324/3. This device, as shown in FIG. 1, has a main shaft 2 rotatably inserted into a main shaft unit 1. At a lower end portion of the main shaft 2, a tool holder 4 is removably clamped by a tool clamping/unclamping device 3. This tool clamping/unclamping device 3 comprises a lock nut 5 screwed to the lower end of the main shaft 2, and a locking mechanism 6 for locking the lock nut 5 in the direction of rotation. The lock nut 5, cylindrically shaped, has a lock hole 5a concavely provided halfway on its outer circumferential face and a plurality of urging pieces 5b inwardly protrusively provided at the lower end of its inner circumferential face.
The locking mechanism 6, comprising a cylinder 7, and a hydraulic feed system 8 for connecting the cylinder 7 to a hydraulic source, is so arranged that a piston rod 9 disposed as inserted within the cylinder 7 is advanced so as to be fitted into the lock hole 5a of the lock nut 5, thereby locking the lock nut 5 in the direction of rotation. In addition, outward of the lock nut 5 is disposed a position sensor 10 for detecting the angular position of the lock nut 5.
The tool holder 4 has an edge tool T fitted thereto. The tool holder 4 also has at its flange portion 4a a plurality of keyways 4b so that the keyways can be fitted to a power transmission key 11 attached to the lower end of the main shaft 2. The flange portion 4a is urged by the urging pieces 5b of the lock nut 5.
On the outer circumferential face of the tool holder 4, a gripping groove 4c is formed into an annular shape and is to be held by an exchange arm, which is not shown.
Next, the operation of tool replacement by this prior-art device is described with reference to FIG. 2.
(1) The gripping groove 4c of the tool holder 4 is gripped by the exchange arm (step S1);
(2) The main shaft 2 is rotated at a low speed and positioned by the position sensor 10 so that the lock hole 5a on the outer circumferential face of the lock nut 5 and the piston rod 9 of the locking mechanism 6 coincide with each other (step S2);
(3) Then, the piston rod 9 of the locking mechanism 6 goes ahead so as to be engaged with the lock hole 5a of the lock nut 5, so that the lock nut 5 is locked in the direction of rotation (step S3);
(4) In this locking state, the main shaft 2 rotates, causing the lock nut 5 to be loosened, so that the notches of the tool holder 4 are engaged with the urging pieces 5b of the lock nut 5 (step S4);
(5) With the tool holder 4 gripped by the exchange arm, the arm goes down, where the tool holder is replaced by a new one, then the arm goes up (steps S5-S7);
(6) When the new tool holder 4 is fitted in the lock nut 5, the main shaft 2 rotates in the direction reverse to that in step S4 with the lock nut 5 locked, causing the lock nut 5 to securely tighten the tool holder 4 (step S8); and
(7) Thereafter, the piston rod 9 of the locking mechanism 6 retreats so as to release the lock nut 5 from locking, and finally the exchange arm returns to the standby position inversely to step S2 (steps S9 and S10).
However, in the above-described prior art, to prevent the tool holder 4 from dropping off by its dead weight when the lock nut 5 is loosened to its removal position, it is arranged that the clamping and unclamping of the tool holder 4 is carried out by rotating the main shaft 2 with the tool holder 4 first gripped by the exchange arm, as understood from operation of tool replacement shown in FIG. 2. Thus, the operation of the exchange arm cannot be exercised in any continuous manner. This would require longer time for the operation of tool replacement, unfavorably.
When there arises a shift in the phase relation between the lock nut 5 and the main shaft 2 with the tool unclamped, it is required to make the power transmission key 11 of the main shaft 2 and the urging pieces 5b of the lock nut 5 coincident in phase by manually rotating the lock nut 5. This phase adjustment work may encounter difficulties.
Besides, when the main shaft 2 is rotated at high speed with the tool unclamped, there would be a possibility that the lock nut 5 might drop off from the main shaft 2 due to excessive rotation of the lock nut 5. Still another problem is that the state of how the tool holder 4 is held by the lock nut 5 cannot be checked during the rotation of the main shaft 2.
On the other hand, whereas the above-described prior art would involve the position sensor 10, the location where the position sensor 10 is disposed is necessarily in proximity to the cutting point from some structural reason. This leads to a possibility that some failure or malfunction may occur due to chips or others. Moreover, in machines of such a type that the main shaft 2 is replaced in the whole unit including its attachments, it would be difficult in some cases to provide electrical wiring required to operate the position sensor 10.
Further, it is arranged that the position of the protrusion of the lock nut 5 is detected by the position sensor 10 while the main shaft 2 is rotated at low speed, thereby accomplishing the alignment between the lock hole 5a and the piston rod 9. As a result, a longer time would be taken for the work of tool replacement.
Yet further, due to the arrangement of the locking mechanism 6 that the piston rod 9 is moved by a hydraulic cylinder or the like, the whole system becomes complex and expensive, adding to the work of maintenance and inspection and the like.
Furthermore, due to the arrangement that the main shaft 2 is rotated by a specified angle, the thickness of the flange portion 4a of the tool holder 4 to be clamped by the lock nut 5 is not equal in all cases of the tool holder 4. Also, due to possible abrasion of the screw over a long term use, a significant variation in the clamping force may take place, such that chatters, edge damage, and the like may occur during machining processes. As a result, the machining precision may be adversely affected to a great extent, unfavorably.
The nut runner type device mentioned before would require a male screw 2a to be provided to screw the lock nut 5 to an end of the main shaft 2. Thus, it cannot be utilized for general purposes, disadvantageously. This type of device also involves a lengthened taper portion 2b, such that the tool holder 4 needs to be moved over a considerable distance to be withdrawn completely from the main shaft 2 during tool replacement. This would result in making it difficult to speed up the operation of automatic tool replacement.